With the increased sale and use of personal media players and now portable smart phones, headphone use continues to increase. Internet radio stations and streaming music and video services provide content at all hours. Users enjoy music, video, and telephone conversations through wired earphones, earbuds, and headphones. Because these devices are portable they, and their corresponding wired headsets are used more often and in many different environments. With a large investment in headsets users are also more prone to use them also with tablets, notebook computers, and many other portable and even fixed devices.
A stereo headphone set, coupled into a mains-powered headphone amplifier in the living room still provides a clear clean audio experience to a careful listener. A lightweight mobile headset coupled to a portable device, on the other hand, may turn out to be unpleasant or even dangerous. The wires of a wired headset not only carry electrical analog power signals to the connected speakers but also act as wire antennas to receive ambient electro-magnetic noise in the environment surrounding the user. The electro-magnetic energy in the ambient is converted to electricity by the headset wires and propagates in both directions within the headset wires. Different headsets couple different amounts and types of noise based on their antenna properties. Their antenna properties comes from their geometry, material properties etc. In other words, the RF (radio frequency) noise coupled into the system causes an RF current in the wires.
The RF noise will travel toward the audio transducers at the user's ears and be coupled into those transducers. The signal level is typically so low that this noise is inaudible. The RF noise will also be coupled into the audio amplifier that is driving audio signals to the audio transducers. In this case, the RF noise is amplified and may even develop a feedback loop. The amplified noise may be annoying to the user and may possibly be loud enough to be a safety risk for the user. While RF noise is typically at frequencies, e.g. 150 kHz to 6 GHz, beyond the range of human hearing, e.g. 20 Hz to 20 kHz, the RF signal in the headset wires can carry a modulating signal that is within the human hearing range. A non-linear audio amplifier typically demodulates the modulated RF noise signal. In so doing it generates a low frequency audio noise signal that is then amplified and provides noise in the headset.